During the operation of gas or oil fields, the recovery of the oil can be improved by injecting water into the deposit, via an injection well, in such a way as to push the oil from the deposit out of the ground, via at least one other well called the production well. The interaction of the injected water with the reservoir containing the gas, the oil and underground water can result in the formation of chemical species able to cause operating defects in the installations. More particularly, deposits of barium sulphate, calcium carbonate or zinc and lead sulphides, for example, are in particular able to form in operating conditions with high pressure and high temperature or during the putting into contact of the injected brine in order to extract the oil or the gas with the hydrogen sulphide or the ions contained in the reservoir. The production of reservoir or aquifer water simultaneously with the oil or the gas can result in the same phenomena. These mineral deposits are able to obstruct the flow channels in the formation, to pollute the pipes and the surface equipment and to block the pumping systems. More generally, mineral deposits or corrosion phenomena can appear in different operating conditions.
In order to prevent or slow these phenomena, additives are injected into the underground formation or into the gas or oil wells. An example of such additives is comprised of mineral deposition inhibitors and of corrosion inhibitors. The injection of a fluid containing an inhibitor capable of preventing the formation of problematic crystals, preventing their growth or dispersing them makes it possible as such to counter the aforementioned disadvantages and to avoid or delay the disassembly of the installations in order to clean them.
The dosage of these inhibitors constitutes however an essential aspect of the production of hydrocarbons, in order to ensure that they are present in sufficient quantities to fulfil their function and, in case of need, to inject in a timely manner an additional quantity of inhibitor, adjusted in order to take into account the economic constraints of the method and its environmental impact.
The methods currently used to dose these inhibitors are often not very accurate and/or long and require equipment that is often insufficiently adapted to the operating conditions. One of the examples of these methods is the dosage of corrosion inhibitors via methylorange. Although this technique has great flexibility, it is cruelly lacking in reliability and has a high degree of relative uncertainty on the results.
For the dosing of molecules that inhibit mineral deposits, a precise chemical analysis is required for a certain number of specific elements of one of the molecules used (measurement of the nitrogen or phosphorus content for example, method referred to as Hyamine for polymers). These measurements can be taken either by mass spectrometry and/or by separation and concentration using analytical devices such as high performance liquid chromatography (HPLC). These techniques are both complex and difficult to set up.
The developing of a reliable and fast method of detection is further hindered by the fact that these inhibitors are generally so effective that they are present in the injected fluid only as a few ppm, and by the presence in the fluid constituted of operating water with a diversity of compounds such as salts and organic residue. Because of the presence of these compounds, the complex fluid produced has in particular an intrinsic fluorescence which prevents the detection of inhibitors, possibly tagged by fluorescent probe, using conventional fluorescence techniques. Finally, the production sites are generally located in remote areas, far from local analysis laboratories, which forms an additional constraint.
It would therefore be desirable to be able to dose these inhibitors directly on the site, in the production water, using a method that is simple, reliable and accurate, that can be used on a diversity of mineral deposition and corrosion inhibitors and that can be implemented using devices that are not large in order to be moved easily.
The inventors have shown that these needs could be satisfied by combining these inhibitors with a lanthanide ion and by using the time-resolved fluorescence method. This method makes it possible indeed to overcome the natural fluorescence of the operating water, which has very short emission times, and to collect only the light emitted after a delay of a few microseconds to one millisecond, more preferably from 100 microseconds to one millisecond, resulting from the fluorescence of the inhibitors tagged as such. The dosage of the inhibitors can then be carried out via quantification of the phosphorescence signal emitted, with precision to less than 10 ppm, and even less than 1 ppm.
This method furthermore has the advantage of specifically identifying the type of inhibitor (of mineral deposition or corrosion), even when it is present in a complex fluid formed by the production water in the oil medium, according to its optical signature, by using simultaneously the excitation and emission spectra and the lifetimes of the signals emitted.